Hi all, I encounter a queueing related challenge and need you guys'
help badly:
The use case is that I have 3 tasks that I want to insert into the
queue. Each task takes about 20 seconds to complete. The challenge is
that task # 3 depends on task # 2, and task # 2 depends on task # 1.
The question is, how can I insert those 3 tasks and expect task 1 be
finished before task 2 starts, and task 2 be finished before task 3
starts?
Base upon my current knowledge about GAE, I am thinking about wrapping
a new transaction around every single task such as follows:
-------------------------------------------------
// run task #1
Queue queue = QueueFactory.getDefaultQueue();
Transaction tx = pm.currentTransaction();
try {
tx.begin();
queue.add(DatastoreServiceFactory.getDatastoreService().getCurrentTransaction(),
TaskOptions().url("/task1.jsp"));
tx.commit();
} finally { if (tx.isActive()) { tx.rollback();} }
// run task #2
tx = pm.currentTransaction();
try {
tx.begin();
queue.add(DatastoreServiceFactory.getDatastoreService().getCurrentTransaction(),
TaskOptions().url("/task2.jsp"));
tx.commit();
} finally { if (tx.isActive()) { tx.rollback();} }
// run task #3
tx = pm.currentTransaction();
try {
tx.begin();
queue.add(DatastoreServiceFactory.getDatastoreService().getCurrentTransaction(),
TaskOptions().url("/task2.jsp"));
tx.commit();
} finally { if (tx.isActive()) { tx.rollback();} }
-------------------------------------------------
where task2.jsp and task3.jsp both perform select, update and insert
operations based upon the datastore contents from the prior tasks (ie,
task 3 depends on task 2, and task 2 depends on task 1).
Is this the right way to implement the synchronous queued tasks? Is
there a better solution out there offered by GAE? Your help is
greatly appreciated and thanks in advance.
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